AIDS, or Acquired Immunodeficiency Syndrome, is caused by the human immunodeficiency virus (HIV) and is characterized by several clinical features including wasting syndromes, central nervous system degeneration and profound immunosuppression that results in opportunistic infections and malignancies. HIV is a member of the lentivirus family of animal retroviruses, which include the visna virus of sheep and the bovine, feline and simian immunodeficiency viruses (SIV). Two closely related types of HIV, designated HIV-1 and HIV-2, have been identified thus far, of which HIV-1 is by far the most common cause of AIDS. However. HIV-2, which differs in genomic structure and antigenicity, causes a similar clinical syndrome.
An infectious HIV particle consists of two identical strands of RNA, each approximately 9.2 kb long, packaged within a core of viral proteins. This core structure is surrounded by a phospholipid bilayer envelope derived from the host cell membrane that also includes virally-encoded membrane proteins (Abbas et al., Cellular and Molecular Immunology, 4th edition, W.B. Saunders Company, 2000, p. 454). The HIV genome has the characteristic 5′-LTR-Gag-Pol-Env-LTR-3′ organization of the retrovirus family. Long terminal repeats (LTRs) at each end of the viral genome serve as binding sites for transcriptional regulatory proteins from the host and regulate viral integration into the host genome, viral gene expression, and viral replication.
The HIV genome encodes several structural proteins. The gag gene encodes structural proteins of the nucleocapsid core and matrix. The pol gene encodes reverse transcriptase (RT), integrase (IN), and viral protease (PR) enzymes required for viral replication. The tat gene encodes a protein that is required for elongation of viral transcripts. The rev gene encodes a protein that promotes the nuclear export of incompletely spliced or unspliced viral RNAs. The vif gene product enhances the infectivity of viral particles. The vpr gene product promotes the nuclear import of viral DNA and regulates G2 cell cycle arrest. The vpu and nef genes encode proteins that down regulate host cell CD4 expression and enhance release of virus from infected cells. The env gene encodes the viral envelope glycoprotein that is translated as a 160-kilodalton (kDa) precursor (gp160) and cleaved by a cellular protease to yield the external 120-kDa envelope glycoprotein (gp120) and the transmembrane 41-kDa envelope glycoprotein (gp41), which are required for the infection of cells (Abbas et al., Cellular and Molecular Immunology, 4th edition, W.B. Saunders Company, 2000, pp. 454-456). gp140 is a modified form of the Env glycoprotein, which contains the external 120-kDa envelope glycoprotein portion and the extracellular part of the gp41 portion of Env and has characteristics of both gp120 and gp41. The nef gene is conserved among primate lentiviruses and is one of the first viral genes that is transcribed following infection. In vitro, several functions have been described, including down-regulation of CD4 and MHC class I surface expression, altered T-cell signaling and activation, and enhanced viral infectivity.
HIV infection initiates with gp120 on the viral particle binding to the CD4 and chemokine receptor molecules (e.g., CXCR4, CCR5) on the cell membrane of target cells such as CD4+ T-cells, macrophages and dendritic cells. The bound virus fuses with the target cell and reverse transcribes the RNA genome. The resulting viral DNA integrates into the cellular genome, where it directs the production of new viral RNA, and thereby viral proteins and new virions. These virions bud from the infected cell membrane and establish productive infections in other cells. This process also kills the originally infected cell. HIV can also kill cells indirectly because the CD4 receptor on uninfected T-cells has a strong affinity for gp120 expressed on the surface of infected cells. In this case, the uninfected cells bind, via the CD4 receptor-gp120 interaction, to infected cells and fuse to form a syncytium, which cannot survive. Destruction of CD4+ T-lymphocytes, which are critical to immune defense, is a major cause of the progressive immune dysfunction that is the hallmark of AIDS disease progression. The loss of CD4+ T cells seriously impairs the body's ability to fight most invaders, but it has a particularly severe impact on the defenses against viruses, fungi, parasites and certain bacteria, including mycobacteria.
Research on the Env glycoprotein has shown that the virus has many effective protective mechanisms with few vulnerabilities (Wyatt & Sodroski, Science. 1998; 280:1884-8). For fusion with its target cells, HIV-1 uses a trimeric Env complex containing gp120 and gp41 subunits (Burton et al., Nat Immunol. 2004; 5:233-6). The fusion potential of the Env complex is triggered by engagement of the CD4 receptor and a coreceptor, usually CCR5 or CXCR4. Neutralizing antibodies seem to work either by binding to the mature trimer on the virion surface and preventing initial receptor engagement events, or by binding after virion attachment and inhibiting the fusion process (Parren & Burton, Adv Immunol. 2001; 77:195-262). In the latter case, neutralizing antibodies may bind to epitopes whose exposure is enhanced or triggered by receptor binding. However, given the potential antiviral effects of neutralizing antibodies, it is not unexpected that HIV-1 has evolved multiple mechanisms to protect it from antibody binding (Johnson & Desrosiers, Annu Rev Med. 2002; 53:499-518).
Most experimental HIV-1 vaccines tested in human and/or non-human primate suggests that a successful vaccine will incorporate immunogens that elicit broad neutralizing antibodies (bNabs) and robust cell-mediated immunity. HIV-1 envelope glycoprotein (Env) is the main viral protein involved in the entry of the virus and is also the primary target for neutralizing antibodies, but due to immune evasion strategies and extreme sequence variability of Envs, generation of bNabs has been a daunting task (Phogat S, Wyatt R. Curr Pharm Des. 2007; 13:213-27, Phogat S, et al. J Intern Med. 2007 262:26-43, Karlsson Hedestam G B, et al. Nat Rev Microbiol. 2008 6:143-55).
The ability to elicit broad and potent neutralizing antibodies is a major challenge in the development of an HIV-1 vaccine. Namely, HIV-1 has evolved an impressive array of strategies to evade antibody-mediated neutralization, bNabs develop over time in a proportion of HIV-1 infected individuals, and a handful of broad neutralizing monoclonal antibodies have been isolated from clade B infected donors. These antibodies tend to display less breadth and potency against non-clade B viruses, and they recognize epitopes on the virus that so far have failed to elicit broad neutralizing responses when incorporated into a diverse range of immunogens. Presumably, this is due to the ability of these bNabs to recognize conserved recessed targets on HIV Env, which are either inaccessible by elicited antibodies or difficult to precisely redesign and present to the immune system.
Recently using a sensitive high-throughput micro-neutralization screening of supernatants from approximately 30,000 IgG+memory B cells from a HIV-1 clade A-infected African donor, two new bNabs, PG9 and PG16, that are broad and exceptionally potent neutralizing antibodies were identified (Walker L, Phogat S, et al. Science. 2009; 326:285-9. Epub 2009 Sep. 3). These antibodies recognize a new conserved, yet accessible, vaccine target (consisting of conserved elements on the variable loops 2 and 3) on the Env and show preferential binding to HIV Env trimers (model of PG9 and PG16 epitopes on HIV-1 trimer.).
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